1. Field of the Invention
This invention relates to a speaker unit for low frequency reproduction, and more particularly to a speaker unit for low frequency reproduction wherein a duct having a port (opening) formed thereon is provided in a speaker enclosure.
2. Description of the Related Art
Various speaker units for low frequency reproduction are conventionally known and include a speaker unit for low frequency reproduction of the phase inversion type as shown in a sectional view and a front elevational view in FIGS. 8(a) and (b), respectively. Referring to FIGS. 8(a) and 8(b), the speaker unit for low frequency reproduction of the phase inversion type shown includes a speaker 4 provided on an enclosure 3 in the form of a box in which a speaker opening 1 is formed. A duct (pipe) 5 having a port (opening) 2 is formed on a front wall of the enclosure 3. A sound absorbing material 6 is suitably disposed in the inside of the enclosure 3. In the speaker unit for low frequency reproduction of the phase inversion type having such a structure as just described, the enclosure (called Vented Type Enclosure) itself can be formed as a Helmholtz resonator and frequencies of the entire lower side frequency band can be reinforced. The reason is described below.
A speaker vibrates forwardly and backwardly to produce waves of compression and rarefaction in a space. When the speaker moves forwardly, the air in front of the cone becomes dense while the air in the rear of the cone becomes rarefied, and the phases of them are just opposite to each other. If the speaker is driven by itself without a baffle, then the compression and rarefaction conditions in the front and rear of the speaker mix with and cancel each other, and no sound is produced. In order to prevent sounds of the opposite phases in the front and rear of the cone from mixing with each other, a large baffle should be provided.
However, a baffle which is effective also with regard to low frequencies requires a corresponding large size and is not practical. Therefore, in a speaker unit for low frequency reproduction of the phase inversion type, the phase of sound from the rear of the cone is inverted by 180° by means of the duct 5 so that the sound may have the same phase as that of the sound in front of the cone, and the sound of the inverted phase is radiated from the port 2. In other words, the speaker unit for low frequency reproduction of the phase inversion type makes positive use of sound radiated from the rear face of the speaker in that a “Helmholtz resonance” action is generated by the internal volume of the enclosure 3 and the duct 5, and sound is radiated to the outside from the port 2 by the resonance action. Since the radiated sound has the same phase as that of sound radiated to the front of the speaker unit, it acts to augment the radiation efficiency just of low sound. The reason why the term “phase inversion” is used is that the enclosure makes positive use of the fact that sound of the reverse phase from the rear face of the speaker unit is converted into sound of the normal phase when it is radiated from the port upon resonance, and the resulting sound promotes the effect of the sound of the normal phase from the front face of the speaker unit.
FIG. 9 is a perspective view of another conventional speaker unit for low frequency reproduction of the phase inversion type. Referring to FIG. 9, in the speaker unit for low frequency reproduction of the phase inversion type shown, a speaker opening 1 and a port 2 are formed in different adjacent faces of an enclosure 3, and a duct 5 having a circular cross section is formed in the enclosure 3.
FIG. 10 is a diagram illustrating an electric impedance characteristic (f-Z characteristic) of a conventional speaker unit for low frequency reproduction of the phase inversion type. It can be seen from the f-Z characteristic illustrated in FIG. 10 that the speaker unit has a first resonance frequency (resonance frequency of the speaker 4 attached to the box) f1 and a second resonance frequency (resonance frequency of the port 2) f2 and has a resonance frequency f0 between the first resonance frequency f1 and the second resonance frequency f2, and exhibits a high radiation efficiency in the proximity of the resonance frequency f0.
FIG. 11 shows an example of an arrangement of speakers in an automobile of the sedan type. Referring to FIG. 11, four speakers S1 to S4 of the full range are disposed on the front and rear, left and right doors of the automobile, and in addition, a speaker S5 for low frequency reproduction of the type described hereinabove with reference to FIG. 8 or 9 is disposed as a sub woofer in the trunk space. In a RV vehicle or an automobile of the mini van type, the speaker S5 for low frequency reproduction is disposed at an end of a bulk storage space at a rear portion.
In a speaker unit for low frequency reproduction of the phase inversion type, the maximum sound pressure is obtained around the resonance frequency. Conventionally, a speaker unit for low frequency reproduction of the phase inversion type is designed such that the first and second resonance frequencies f1 and f2 (FIG. 10) may both have suitable values within an actually used band (about 20 Hz to 100 Hz). Since the electric impedance varies around the first and second resonance frequencies f1 and f2, the frequency phase characteristic (f-θ characteristic) is not flat at the phase=0 as seen from the broken line in FIG. 10. In other words, in a conventional speaker unit for low frequency reproduction, the phase θ is retarded or advanced in the actually used band, and a delay in time is generated depending upon the frequency f. Conventionally, it is desirable that sound arrives in a state of zero phase delay, and if the phase is retarded or advanced depending upon the frequency, then the sound becomes impure and distortion of the sound increases. Since particularly the sub woofer S5 is disposed at a location farthest from the driver's seat in the automobile as shown in FIG. 11, the delay of the sound increases and a distortion in sound quality is generated by phase displacements from the main speakers S1 to S4. Further, a problem occurs that the orientation of a sound image of the sub woofer S5 is deteriorated.